Method and Apparatus for Directional Centralized Contention Based Period in a Wireless Communication System

ABSTRACT

A method of communication includes allocating a portion of a superframe centralized contention based period where the access method is based on directional ALOHA. The centralized contention based period is divided into equal time slots, and each sequential set of N time slots forms a time cycle. During a time cycle, a wireless device listens for requests from other wireless devices while it changes its receiving direction from one time slot to another.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/911,732, filed Oct. 26, 2010, which claims priority under 35 U.S.C.119(e) to U.S. Provisional Application Ser. No. 61/259,621, filed Nov.9, 2009.

FIELD

Certain aspects of the present disclosure relate to wirelesscommunication, and particularly, to directional channel access in awireless communication system.

BACKGROUND

In one aspect of the related art, a wireless communication systemcomprises a set of devices supporting at least one of a single-carrier(SC) physical (PHY) layer and an Orthogonal Frequency DivisionMultiplexing (OFDM) physical layer may be used for millimeter wavecommunications, such as the systems envisioned in the Institute ofElectrical and Electronic Engineers (IEEE) 802.11.ad and IEEE 801.15.3cstandards, and the Wireless Gigabit Alliance (WGA). The PHY layer may beconfigured for millimeter wave communications in the spectrum of 57 to66 gigahertz (GHz), or Ultra Wide Band (UWB) communications in thespectrum of 3.1 to 10.6 GHz.

To allow interoperability between devices or networks that supporteither single-carrier or OFDM PHY modes, all devices further support acommon mode referred to as a control PHY. Specifically, the common modeis a single-carrier base-rate mode employed by both OFDM andsingle-carrier devices to facilitate co-existence and interoperabilitybetween different devices and different networks. The common mode may beemployed for beaconing, control, management, and communicating commandand data frames (packets).

In another aspect of the related art, devices typically employ one ormore Golay codes to provide spreading of different fields of a packet.Complementary codes, first introduced by Golay in M. Golay,“Complementary Series,” IRE Transaction on Information Theory, Vol. 7,Issue 2, April 1961, are sets of complementary pairs of equally long,finite sequences of two kinds of elements. These complementary pairshave the property that the number of pairs of like elements with anygiven separation in one code is equal to the number of unlike elementswith the same separation in the other code. The complementary codesfirst described by Golay were pairs of binary complementary codes withelements +1 and −1, wherein the sum of their respective aperiodicautocorrelation sequence is zero everywhere, except for the center tap.

In a wireless network, such as a wireless personal area network (WPAN)or a wireless local area network (WLAN), devices typically use a slottedALOHA protocol or a carrier sense multiple access/collision avoidance(CSMA/CA) protocol to access the wireless medium. However, these accessmethods do not perform well when one or more devices use directionalantenna patterns for their transmissions and/or receptions.

Therefore, there is a need in the art for a directional channel accessprotocol for devices that may have directional antenna systems, such asphased antenna arrays, directional antennas, or sectored antennas.

SUMMARY

Aspects disclosed herein may be advantageous to systems employingmillimeter-wave WPANs or WLANs (such as the WLANs described by theIEEE802.11.ad, IEEE 802.11.ac and WGA protocols). However, thedisclosure is not intended to be limited to such systems, as otherapplications may benefit from similar advantages.

According to an aspect of the disclosure, a superframe allocated by afirst wireless device contains a centralized contention period and adistributed contention period. During the centralized contention period,the first device is part of any communication link between a pair ofwireless devices. The distributed contention period may be used forpeer-to-peer communications between wireless devices. The distributedcontention period may be used for communication between the first deviceand at least one other wireless device.

According to another aspect of the disclosure, a superframe allocated bya first wireless device comprises a centralized contention period thatis further divided into fixed equal-size time slots. The first devicechanges its receive antenna pattern (also referred to as its direction)from one time slot to another in a cyclic manner. Specifically, thefirst device uses a first receive direction in the first time slot, asecond receive direction in the second time slot, and an N^(th) receivedirection in the N^(th) time slot. The first device reuses its firstreceive direction in the (N+1)^(th) time slot, its second receivedirection in the (N+1)^(th) time slot, its N^(th) receive direction inthe (2N)^(th) time slot, etc.

According to another aspect of the disclosure, a method of communicationis provided for accessing the centralized contention period by one ormore other wireless devices (e.g., a second wireless device) tocommunicate with a first wireless device (a master device) using adirectional slotted ALOHA protocol. The second device may transmit aframe on a time-slot boundary using a transmit antenna pattern selectedfrom a predetermined set of transmit antenna patterns. The second devicewaits for a response from the first device. The first device cyclesthrough its different receive patterns (i.e., directions) in each timeslot.

According to another aspect of the disclosure, a method of communicationis provided for allowing access to the centralized contention period byone or more wireless devices (e.g., a second wireless device) thatcommunicate with a first wireless device (e.g., a master device) using adirectional slotted ALOHA protocol. The second device maintains a set ofN back-off window sizes equal in number to the first device's number Nof receive directions. The second device may draw a set of N randomnumbers between one and the back-off window size(s). Each random numberindicates a particular time cycle and time slot within the time cycle,which provides a period of time that the second device waits beforetransmitting the frame. Each time cycle comprises N time slots, and theselection of a time slot in a time cycle may be determined by the randomnumber index in the set of the N random numbers.

According to another aspect of the disclosure, a method of communicationis provided for accessing a centralized contention period that is usedto communicate with a first wireless device (a master device) using adirectional cycle-based ALOHA protocol. A second wireless devicetransmits a frame in the first time slot of a time cycle using one of aplurality of transmit antenna patterns from a set and waits for aresponse from the first device, which uses a first receive direction inthe first time slot. The second device transmits the frame in the secondtime slot of a time cycle using the same transmit antenna pattern andwaits for a response from the first device, which employs a secondreceive direction in the second time slot. The second device employssuccessive (e.g., sequential) time slots of a time cycle fortransmitting the frame until it successfully decodes a response backfrom the first device, or until it has transmitted the frame in all Ntime slots.

According to another aspect of the invention, the centralized contentionperiod may be used for authentication, association, service periodrequests, data communications, and/or direction acquisition and trackingEach time slot has a fixed duration, the time slot duration being atleast equal to the duration of a transmit request frame, a first guardperiod (commonly known as an SIFS (Short Inter Frame Spacing)), theduration of a response frame, and a second guard period (e.g., a secondSIFS).

According to another aspect of the disclosure, a communications methodcomprises transmitting a service period request (also known as a channeltime allocation request) from a second wireless device to a firstwireless device, wherein the service period request is transmitted usingthe directional slotted ALOHA protocol; receiving a service periodallocation granted by the first device and transmitted using a secondtransmit pattern; and transmitting at least one frame from the seconddevice to a destination device in the service period.

According to another aspect of the disclosure, a method of communicationcomprises employing at least one of a full double sweep and a partialdouble sweep for finding a pair of downlink working directions.

The partial double sweep comprises transmitting a set of request framesone at a time using a directional ALOHA protocol, in a first transmitdirection from a second wireless device to a first wireless device. Thefirst device changes its receive direction from one time slot to anotherin a cyclic manner (i.e., the first device repeats the same N receivedirections in each time cycle). The second device listens for a responsefrom the first device. If no response is detected, the second devicesends a set of request frames using a second transmit direction one at atime using the directional ALOHA protocol, and the process of sendingrequest frames and listening for a response may be repeated for up toall possible transmit directions or until the second device successfullydetects a response from the first device. The second device uses thedirection(s) for which it successfully decoded a response from the firstdevice as a working direction(s) that it uses for further communicationswith the first device.

The full double sweep comprises transmitting a set of request frames,one at a time using the directional ALOHA protocol, in a first directionfrom the second device to the first device. The first device changes itsreceive direction from one time slot to another in a cyclic manner. Thesecond device sends another set of request frames in a second transmitdirection to the first device, one at a time using the directional ALOHAprotocol. The process of sending request frames is repeated for alltransmit directions of the second device. The second device selects thedirection(s) with the highest link quality indicator (LQI) as apreferred direction(s) for communicating with the first device.

In accordance with one aspect of the invention, a wireless systemcomprises means for selecting a sequence of time slots paired with aplurality of transmit directions for transmitting at least one requestframe from a first wireless device to a second wireless device; meansfor listening for the at least one request frame at the second wirelessdevice by employing a different one of a plurality of receive directionsin each of the time slots; means for transmitting at least one responseframe from the second wireless device to the first wireless device;means for listening for the at least one response frame at the firstwireless device; and means for selecting a preferred set of uplink anddownlink directions for further communication between the first wirelessdevice and the second wireless device. Means for selecting the sequenceof time slots may include, by way of example, but without limitation, adigital computer system comprising a memory for storing instructions anda processor for executing the instructions. Means for listening mayinclude, by way of example, but without limitation, any wireless radioreceiver employing directional beam patterns and configured to detect,demodulate, and/or decode received transmissions. Means for transmittingmay include, by way of example, but without limitation, any wirelessradio transmitter employing directional beam patterns and configured forgenerating a response frame and other data signals, and coupling datasignals into a wireless communication channel. Means for selecting apreferred set of uplink and downlink directions may include, by way ofexample, but without limitation, a digital computer system comprising amemory for storing instructions and a processor for executing theinstructions, and may share one or more components used by the means forselecting the sequence of time slots.

In accordance with another aspect of the invention, a wireless devicecomprises means for selecting a sequence of time slots paired with aplurality of transmit directions for transmitting at least one requestframe from the first wireless device to a second wireless device, thesecond wireless device employing a different one of a plurality ofreceive directions for each of the time slots; means for listening forat least one response frame transmitted by the second wireless device;and means for selecting a preferred set of uplink and downlinkdirections for further communication between the first wireless deviceand the second wireless device. Means for selecting the sequence of timeslots may include, by way of example, but without limitation, a digitalcomputer system comprising a memory for storing instructions and aprocessor for executing the instructions. Means for listening mayinclude, by way of example, but without limitation, any wireless radioreceiver employing directional beam patterns and configured to detect,demodulate, and/or decode received transmissions. Means for selecting apreferred set of uplink and downlink directions may include, by way ofexample, but without limitation, a digital computer system comprising amemory for storing instructions and a processor for executing theinstructions, and may share one or more components used by the means forselecting the sequence of time slots.

In accordance with another aspect of the invention, a wireless devicecomprises means for employing a different one of a plurality of receivedirections for each of a sequence of time slots to listen for at leastone request frame transmitted by a second wireless device; means fortransmitting at least one response frame to the second wireless devicein response to a received request frame; and means for selecting apreferred set of uplink and downlink directions for furthercommunication between the first wireless device and the second wirelessdevice. Means for employing a different one of a plurality of receivedirections for each of a sequence of time slots to listen for at leastone request frame transmitted by a second wireless device may include,by way of example, but without limitation, any wireless radio receiveremploying directional beam patterns and configured to detect,demodulate, and/or decode received transmissions. Means for transmittingmay include, by way of example, but without limitation, any wirelessradio transmitter employing directional beam patterns and configured forgenerating a response frame and other data signals, and coupling datasignals into a wireless communication channel. Means for selecting apreferred set of uplink and downlink directions may include, by way ofexample, but without limitation, a digital computer system comprising amemory for storing instructions and a processor for executing theinstructions.

In accordance with another aspect of the invention, a wireless devicecomprises means for generating a plurality of time cycle numbers, theplurality of time cycle numbers being equal to a plurality of receivedirections employed by a second wireless device, each of the time cyclenumbers being associated with one of the receive directions and having avalue within a predetermined back-off window size; means forsequentially organizing the plurality of time cycle numbers with respectto their values for producing a sequence of time cycle numbers; meansfor generating a sequence of time slot numbers from the sequence of timecycle numbers and the plurality of receive directions, the sequence oftime slot numbers being used to select time slots for transmitting aframe to the second wireless device. Means for generating the pluralityof time cycle numbers, means, means for sequentially organizing theplurality of time cycle numbers, and means for generating the sequenceof time slot numbers may include, by way of example, but withoutlimitation, a digital computer system comprising a memory for storinginstructions and a computer processor for executing the instructions.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentdisclosure can be understood in detail, a more particular description,briefly summarized above, may be had by reference to aspects, some ofwhich are illustrated in the appended drawings. It is to be noted,however, that the appended drawings illustrate only certain typicalaspects of this disclosure and are therefore not to be consideredlimiting of its scope, for the description may admit to other equallyeffective aspects.

FIG. 1 illustrates a wireless communication system in accordance withcertain aspects of the present disclosure.

FIG. 2 illustrates various components that may be utilized in a wirelessdevice in accordance with certain aspects of the present disclosure.

FIG. 3 illustrates an example transceiver that may be used within awireless communication system in accordance with certain aspects of thepresent disclosure.

FIG. 4A illustrates a conventional superframe structure in accordancewith certain aspects of the present disclosure.

FIG. 4B illustrates a frame structure in accordance with certain aspectsof the present disclosure.

FIG. 5A illustrates a directional beacon period in accordance withcertain aspects of the present disclosure.

FIG. 5B illustrates a contention period using a slotted ALOHA protocolaccording to certain aspects of the present disclosure.

FIG. 6A illustrates a superframe structure in accordance with certainaspects of the present disclosure.

FIG. 6B illustrates a centralized contention period using a directionalALOHA protocol according to certain aspects of the present disclosure.

FIG. 7A illustrates a time-slot structure in accordance with certainaspects of the present disclosure.

FIG. 7B illustrates a time-cycle structure in accordance with certainaspect of the present disclosure.

FIG. 8 illustrates a directional slotted ALOHA protocol in accordancewith certain aspect of the present disclosure.

FIG. 9A illustrates an operation for transmitting a beacon frame thatmay be used within a wireless communication system in accordance withcertain aspects of the present disclosure.

FIG. 9B illustrates components configured for performing the operationsillustrated in FIG. 9A.

FIG. 9C illustrates operations for processing a beacon frame at thereceiver in accordance with certain aspects of the present disclosure.

FIG. 9D illustrates components configured for performing the operationsillustrated in FIG. 9C.

FIG. 10A illustrates a method for performing a directional slotted-ALOHAprotocol with respect to certain aspects of the present disclosure.

FIG. 10B illustrates components of a system configured for performingthe method illustrated in FIG. 10A.

FIG. 10C illustrates a method for processing a frame at the receiver inaccordance with certain aspects of the present disclosure.

FIG. 10D illustrates an apparatus configured for performing the methodillustrated in FIG. 10C.

FIG. 11A illustrates a method for retransmitting a frame using adirectional slotted ALOHA protocol in accordance with certain aspects ofthe present disclosure.

FIG. 11B illustrates an apparatus configured for performing the methodillustrated in FIG. 11A.

FIG. 12A illustrates a method for processing a frame transmitted using adirectional slotted ALOHA protocol at a receiver in accordance withcertain aspects of the present disclosure.

FIG. 12B illustrates an apparatus configured for performing the methodillustrated in FIG. 12A.

FIG. 13A illustrates a method for performing a double sweep using adirectional slotted ALOHA protocol in accordance with certain aspects ofthe present disclosure.

FIG. 13B illustrates an apparatus configured for performing the methodillustrated in FIG. 13A.

DETAILED DESCRIPTION

Various aspects of the disclosure are described more fully hereinafterwith reference to the accompanying drawings. This disclosure may,however, be embodied in many different forms and should not be construedas limited to any specific structure or function presented throughoutthis disclosure. Rather, these aspects are provided so that thisdisclosure will be thorough and complete, and will fully convey thescope of the disclosure to those skilled in the art. Based on theteachings herein one skilled in the art should appreciate that the scopeof the disclosure is intended to cover any aspect of the disclosuredisclosed herein, whether implemented independently of or combined withany other aspect of the disclosure. For example, an apparatus may beimplemented or a method may be practiced using any number of the aspectsset forth herein. In addition, the scope of the disclosure is intendedto cover such an apparatus or method which is practiced using otherstructure, functionality, or structure and functionality in addition toor other than the various aspects of the disclosure set forth herein. Itshould be understood that any aspect of the disclosure disclosed hereinmay be embodied by one or more elements of a claim.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any aspect described herein as “exemplary”is not necessarily to be construed as preferred or advantageous overother aspects.

Although particular aspects are described herein, many variations andpermutations of these aspects fall within the scope and spirit of thedisclosure. Although some benefits and advantages of the preferredaspects are mentioned, the scope of the disclosure is not intended to belimited to particular benefits, uses, or objectives. Rather, aspects ofthe disclosure are intended to be broadly applicable to differentwireless technologies, system configurations, networks, and transmissionprotocols, some of which are illustrated by way of example in thefigures and in the following description of the preferred aspects. Thedetailed description and drawings are merely illustrative of thedisclosure rather than limiting, the scope of the disclosure beingdefined by the appended claims and equivalents thereof. AN EXAMPLEWIRELESS COMMUNICATION SYSTEM

The techniques described herein may be used for various broadbandwireless communication systems, including communication systems that arebased on a single carrier transmission and OFDM. Aspects disclosedherein may be advantageous to systems employing Ultra Wide Band (UWB)signals including millimeter-wave signals, Code Division Multiple Access(CDMA) signals, and OFDM. However, the present disclosure is notintended to be limited to such systems, as other coded signals maybenefit from similar advantages.

FIG. 1 illustrates an exemplary wireless communication system 100 inwhich aspects of the present disclosure may be employed. The wirelesscommunication system 100 may be a broadband wireless communicationsystem compatible with the IEEE 802.11 and 802.15. The wirelesscommunication system 100 may provide communication for a number of BasicService Sets (BSSs) 102, each of which may be serviced by a ServiceAccess Point (SAP) 104. A SAP 104 may be a fixed station or a mobilestation that communicates with Stations (STAs) 106. A BSS 102 mayalternatively be referred to as cell, piconet or some other terminology.The SAP 104 may alternatively be referred to as a base station, apiconet controller, a Node B, a wireless device, a master device, orsome other terminology.

FIG. 1 depicts various stations 106 dispersed throughout the system 100.The stations 106 may be fixed (i.e., stationary) or mobile. Each STA ofthe plurality of STAs 106 implements a MAC and PHY interface to thewireless medium of the network 100. The STAs 106 may alternatively bereferred to as remote stations, access terminals, terminals, subscriberunits, mobile stations, wireless devices, user equipment, etc. The STAs106 may be wireless devices, such as cellular phones, personal digitalassistants (PDAs), handheld devices, wireless modems, laptop computers,personal computers, etc.

Under IEEE 802.11 and 802.15, one STA assumes the role of a coordinator(master) of the BSS. This coordinating STA is referred to as a ServiceAccess Point (SAP) and is illustrated in FIG. 1 as the SAP 104. Thus,the SAP 104 may include the same station functionality of the pluralityof other stations (STAs 106), but provides coordination and managementfor the network. For example, the SAP 104 provides services, such asbasic timing for the network 100 using a beacon; and management of anyQuality of Service (QoS) requirements, power-save modes, and networkaccess control. A wireless device with similar functionality asdescribed for the SAP 104 in other systems may be referred to as anpiconet controller, a base station, a base transceiver station, astation, a terminal, a node, an access terminal acting as an accesspoint, or some other suitable terminology. The SAP 104 coordinates thecommunication between the various stations (STAs 106) in the network 100using a frame structure referred to as a superframe. Each superframe isbounded in time by beacon periods. The SAP 104 may be coupled to asystem controller to communicate with other networks or other SAPs.

A variety of algorithms and methods may be used for transmittinginformation in the wireless communication system 100 between the SAPs104 and the STAs 106 and between the STAs 106 themselves. For example,signals may be communicated between the SAPs 104 and the STAs 106 inaccordance with a CDMA technique and signals may be sent and receivedbetween STAs 106 in according with an OFDM technique. If this is thecase, the wireless communication system 100 may be referred to as ahybrid CDMA/OFDM system.

A communication link that facilitates transmission from an SAP 104 to anSTA 106 may be referred to as a downlink (DL) 108, and a communicationlink that facilitates transmission from an STA 106 to an SAP 104 may bereferred to as an uplink (UL) 110. Alternatively, a downlink 108 may bereferred to as a forward link or a forward channel, and an uplink 110may be referred to as a reverse link or a reverse channel. When two STAscommunicate directly with each other, a first STA will act as the masterof the link, and the link from the first STA to the second STA will bereferred to as the downlink 112, and the link from the second STA to thefirst STA will be referred to as the uplink 114.

A BSS 102 may be divided into multiple sectors. A sector 116 is aphysical coverage area within the BSS 102. SAPs 104 within the wirelesscommunication system 100 may utilize antennas that concentrate the flowof power within a particular sector 116 of the BSS 102. Such antennasmay be referred to as directional antennas.

FIG. 2 illustrates various components that may be utilized in a wirelessdevice 210 employed within the wireless communication system 100. Thewireless device 210 is an example of a device that may be configured toimplement the various methods described herein. The wireless device 202may be an SAP 104 or an STA 106.

The wireless device 202 may include a processor 204 that controlsoperation of the wireless device 202. The processor 204 may also bereferred to as a central processing unit (CPU). Memory 206, which mayinclude one or both read-only memory (ROM) and random access memory(RAM), provides instructions and data to the processor 204. A portion ofthe memory 206 may also include non-volatile random access memory(NVRAM). The processor 204 typically performs logical and arithmeticoperations based on program instructions stored within the memory 206.The instructions in the memory 206 may be executable to implement themethods described herein.

The wireless device 202 may also include a housing 208 that may includea transmitter 210 and a receiver 212 to allow transmission and receptionof data between the wireless device 202 and a remote location. Thetransmitter 210 and receiver 212 may be combined into a transceiver 214.An antenna 216 may be attached to the housing 208 and electricallycoupled to the transceiver 214. The wireless device 202 may include oneor more wired peripherals 224 such as USB, HDMI, or PCIE. The wirelessdevice 202 may also include (not shown) multiple transmitters, multiplereceivers, multiple transceivers, and/or multiple antennas.

The wireless device 202 may also include a signal detector 218 that maybe used to detect and quantify the level of signals received by thetransceiver 214. The signal detector 218 may detect such signals astotal energy, energy per subcarrier per symbol, power spectral density,and/or other signal measurements that are known in the art. The wirelessdevice 202 may also include a digital signal processor (DSP) 220 forprocessing signals.

The various components of the wireless device 202 may be coupledtogether by a bus system 222, which may include a power bus, a controlsignal bus, and a status signal bus, in addition to a data bus.

FIG. 3 illustrates an exemplary transmitter 302 that may be used withina wireless communication system 100 that utilizes CDMA or some othertransmission technique. Portions of the transmitter 302 may beimplemented in the transmitter 210 of a wireless device 202. Thetransmitter 302 may be implemented in a base station 104 fortransmitting data 330 to a user terminal 106 on a downlink 108. Thetransmitter 302 may also be implemented in a station 106 fortransmitting data 330 to a service access point 104 on an uplink 110.

Data 306 to be transmitted are shown being provided as input to aforward error correction (FEC) encoder 308. The FEC encoder 308 encodesthe data 306 by adding redundant bits. The FEC encoder 308 may encodethe data 306 using a convolutional encoder, a Reed Solomon encoder, aTurbo encoder, a low density parity check (LDPC) encoder, etc. The FECencoder 308 outputs an encoded data stream 310. The encoded data stream310 is input to a mapper 314. The mapper 314 may map the encoded datastream onto constellation points. The mapping may be done using somemodulation constellation, such as binary phase-shift keying (BPSK),quadrature phase-shift keying (QPSK), 8 phase-shift keying (8PSK),quadrature amplitude modulation (QAM), constant phase modulation (CPM),etc. Thus, the mapper 312 may output a symbol stream 314, which mayrepresents one input into a block builder 310. Another input in theblock builder 310 may include one or more spreading codes produced by aspreading code generator 318.

The block builder 310 may be configured for partitioning the symbolstream 314, into sub-blocks and creating OFDM/OFDMA symbols orsingle-carrier sub-blocks. The block builder 310 may append eachsub-block with a guard interval, a cyclic prefix, or a spreadingsequence from the spreading codes generator 318. Furthermore, thesub-blocks may be spread by one or multiple spreading codes from thespreading code generator 318.

Output signal 320 may be pre-pended by a preamble 322 generated from oneor more spreading sequences from the spreading code generator 324. Theoutput stream 326 may then be converted to analog and up-converted to adesired transmit frequency band by a radio frequency (RF) front end 328,which may include a mixed signal section and an analog section. Anantenna 330 transmits the resulting signal 332.

FIG. 3 illustrates an exemplary receiver 304 that may be used within awireless device 202 that uses CDMA and/or OFDM/OFDMA. Portions of thereceiver 304 may be implemented in the receiver 212 of a wireless device202. The receiver 304 may be implemented in a station 106 for receivingdata 306 from a service access point 104 on a downlink 108. The receiver304 may also be implemented in a base station 104 for receiving data 306from a user terminal 106 on an uplink 110.

The transmitted signal 332 is shown traveling over a wireless channel334. When a signal 332′ is received by an antenna 330′, the receivedsignal 332′ may be down-converted to a baseband signal by an RFfront-end 328′ which may include a mixed signal and an analog portion.Preamble detection and synchronization component 322′ may be used toestablish timing, frequency and channel synchronization using one ormultiple correlators that correlate with one or multiple spreading codesgenerated by a spreading code generator 324′.

The output of the RF front end block 328′ is input to the frequency andtiming correction component 326′ along with the synchronizationinformation from component 322′. The outputs from components 326′ and322′ are input to a block detection component 316′. When OFDM/OFDMA isused, the block detection may include cyclic prefix removal and fastFourier transform (FFT). When single-carrier transmissions are used, theblock detection may include de-spreading and equalization.

A de-mapper 312′ may perform the inverse of the symbol mapping operationperformed by the mapper 312, thereby outputting soft and/or harddecisions 310′. The soft and/or hard decisions 310′ are input to the FECdecoder 308′, which provides a stream of data estimates 306′. Ideally,this data stream 306′ corresponds to the data 306 that was input to thetransmitter 302.

The wireless system 100 illustrated in FIG. 1 may be a UWB/millimeterwave system operating in the band including the 57-64 GHz unlicensedband specified by the Federal Communications Commission (FCC).

Superframe Structure

FIG. 4A illustrates a superframe 400 used for SAP timing in the network100. In general, a superframe is a basic time-division structurecontaining a beacon period (BP) 410, a contention based period (CBP)420, and a channel time allocation (CTAP) period 430, also known asservice periods (SPs). The superframe is also known as the beacon time(BT) or beacon interval (BI). In the superframe 400, a beacon period(BP) 410 is provided during which an SAP such as the SAP 104 sendsbeacon frames.

A Contention Based Period (CBP) 420 is used to communicate command,control, management, and data frames either between the SAP 104 and atleast one of the plurality of STAs 106 in the network 100, or betweenany set of STAs 106 in the network 100. The access method for the CBP420 may be based on a slotted ALOHA or a carrier sense multiple accesswith collision avoidance (CSMA/CA) protocol.

A Channel Time Allocation Period (CTAP) 430, which is based on a TimeDivision Multiple Access (TDMA) protocol, is provided by the SAP 104 toallocate time for the plurality of STAs 106 to use the channels in thenetwork 100. Specifically, the CTAP is divided into one or more timeperiods (of different sizes), referred to as Channel Time Allocations(CTAs). The CTAs, also known as Service Periods (SPs), are typicallyallocated by the SAP 104 to pairs of stations, one pair of stations to aCTA. Thus, the access mechanism for CTAs is TDMA-based.

FIG. 4B shows an exemplary frame structure 450 that may be used for asingle-carrier, OFDM, or common-mode frame. As used herein, the term,frame, may also be used interchangeably with the term, packet. The framestructure 450 includes a preamble 452, a header 454, and a packetpayload 456. The common mode may use Golay codes for all three fields,i.e., for the preamble 452, the header 454 and the packet payload 456.The common-mode signal uses Golay spreading codes with chip-level/2-BPSKmodulation to spread the data therein. The header 454, which is aphysical layer convergence protocol (PLCP) conforming header, and thepacket payload 456, which is a physical layer service data unit (PSDU),includes symbols spread with a Golay code of length 32 or 64. Variousframe parameters, including, by way of example, but without limitation,the number of Golay-code repetitions and the Golay-code lengths, may beadapted in accordance with various aspects of the frame structure 450.In one aspect, Golay codes employed in the preamble may be selected fromlength-64 or length-128 Golay codes. Golay codes used for data spreadingmay comprise length-32 or length-64 Golay codes.

Referring to FIG. 4B, the preamble 452 includes a packet sync sequencefield 458, an optional start frame delimiter (SFD) field 460, and achannel-estimation sequence field 462.

The packet sync sequence field 458 is a repetition of ones (or arepetition of minus ones, or an alternating sequence of ones and minusones) spread by one of the length-128 complementary Golay codes (a128,b128) as represented by codes 464-1 to 464-Q in FIG. 4B. The SFD field460 comprises a specific code such as {−1, +1} that is spread by one ofthe length-128 complementary Golay codes (a128, b128), as represented bycodes 466-1 and 466-2 in FIG. 4B. The CES field 462 may be spread usinga pair of length-256 complementary Golay codes (a256, b256), such asrepresented by codes 458-1 and 458-2, and may further comprise a cyclicpostfix 458-3, which is a length-128 Golay code. The CES field 462 mayfurther comprise a cyclic prefix (not shown), where CP may be used as aCyclic Prefix or Postfix. A cyclic postfix for code a256 (depicted by458-1) is shown as CP128 458-3 and is a copy of the first 128 chips ofa256 (458-1). The sync field 458 is typically used for AGC (AutomaticGain Control) setting, antenna selection, DC offset removal, packetdetection, timing, and frequency and channel acquisition. The SFD field460 may be used to indicate the end of the sync field 458. The CES field462 is typically used for multipath channel estimation and finefrequency estimation.

In one aspect of the disclosure, a dual-mode millimeter wave systememploying single-carrier modulation and OFDM is provided with asingle-carrier common-mode signaling. The common mode, also known ascontrol PHY (Physical layer), is a single-carrier mode used by bothsingle-carrier and OFDM devices for beaconing, signaling (such ascontrol and management), beamforming, and base-rate data communications.

Directional Aloha Protocol

In typical systems, an SAP transmits a single beacon frame in the beaconperiod (BP) 410, such as depicted in FIG. 4A. This is the case in IEEE802.11 where a SAP transmits the beacon using a single antenna patternthat covers the region of space of interest. In millimeter wave systems,such as the systems being considered in IEEE 802.11 ad and WGA, stationsmay not be omni-capable on transmission or reception. That is, stationsmight not be able to cover the region of space of interest in a singletransmission or reception pattern. Such stations are referred to hereinas directional stations. An omni-capable device is a special case of adirectional station with a single direction. Directional STAs includeSTAs that use switched antennas, sectored antennas, and/or phasedantenna arrays. In what follows, antenna patterns are referred to asdirections, and it should be understood that an antenna direction (e.g.,an antenna pattern) does not necessarily imply a specific geometriccoverage, such as antenna beams. An antenna direction (e.g., a pattern)may take any three-dimensional geometric shape, including, but notlimited to, typical beam and sectored patterns. Furthermore, a stationmay use different directions for transmission and reception.

According to one aspect of the invention, during the beacon interval 410in FIG. 4A, a directional SAP transmits a plurality of beacon framesusing a plurality of transmit antenna patterns, denoted by indices #1 to#M. This is further illustrated in FIG. 5A, wherein an SAP with Mtransmit patterns may transmit M beacon frames, wherein the first beaconframe 510-1 is transmitted in a first transmit direction #1, and thesecond beacon frame 510-2 is transmitted in a second transmit direction#2, and the M^(th) beacon frame 510-M is transmitted in a M^(th)transmit direction #M. The beacon packets may be separated by MinimumInterFrame Spacing (MIFS) guard intervals, such as represented by 520-1to 520-M. Each beacon frame contains one or two counters (typically inthe header of the frame) containing information about the index of thecurrent beacon frame and the total (or remaining) number of beaconframes M. With the exception of the content of the counters, the contentof all beacon frames may be identical.

For an SAP that is omni-capable on transmission (i.e., an SAP with asingle antenna pattern covering the region of interest), M=1. For an SAPwith sectorized antennas, M is the number of sectors that the SAP isable to support. Similarly, when an SAP is provided with switchingtransmit diversity antennas, M may represent the number of transmitantennas in the SAP. Various approaches to the structure of the Q-omnibeacon frame may be used.

The following disclosure relates to the general case of stations (STAs),including SAPs, having transmit directions and receive directions thatmay be different (referred to as asymmetric STAs). Stations havingidentical transmit directions and receive directions (referred to assymmetric STAs) are a special case.

As discussed above, a SAP broadcasts a set of M beacon frames, typicallyin every superframe. Each beacon frame contains all timing informationabout the superframe and, optionally, information about some or all ofthe STAs that are members of the BSS, including the beamformingcapabilities of each STA. The STA beamforming capabilities are obtainedby the SAP during association. An STA beamforming capability includes anumber of transmit and receive directions. An STA may use a differentnumber of transmit and receive directions for different tasks. Forexample, the number of directions could be a number of antennas for anSTA with switched antennas, a number of sectors for an STA with sectoredantennas, or a number of coarse patterns for an STA with a phase antennaarray. A phased antenna array can generate a set of patterns that mayoverlap, each pattern covering a part of the region of the space ofinterest.

The following notation is used to clarify different aspects of thedisclosure. Let M and N be the total number of SAP's transmit andreceive antenna patterns, respectively, and let P and Q be the totalnumber of an STA's transmit and receive antenna patterns respectively.As mentioned above, the number of directions M, N, P, and Q may bechanged. As an example, an STA may use P=2 directions during associationand P=16 directions in a CTAP. Furthermore, an STA may initially use acoarse number of broad directions and adapt either or both thedirections and the number of directions to provide a set of finedirections.

An STA may perform the following steps in order to associate (i.e.,become a member of the BSS) with the SAP. First, the STA searches for abeacon from the SAP. The STA then detects at least one of the Mdirectional beacon frames and acquires knowledge of the superframetiming, the number of the SAP's transmit and receive directions (i.e. Mand N), duration of the CBP, and, optionally, the possible capabilitiesof each of the STA members. In an aspect of the disclosure, the STAacquires and tracks the best SAP transmit direction by measuring a linkquality indicator (LQI) from all K directional beacon packetstransmitted by the SAP. In one aspect of the disclosure, the LQI is ametric of the quality of the received signal. Examples of an LQIinclude, but are not limited to, an RSSI (Received Signal StrengthIndicator), an SNR (Signal to Noise Ratio), an SNIR (Signal to Noise andInterference Ratio), an SIR (Signal to Interference Ratio), a preambledetection, a BER (Bit Error Rate), and a PER (Packet Error Rate).

According to one aspect of the disclosure, an STA may detect a beaconpacket by sweeping over its set of N receive directions over one or moresuperframes. Upon detection of at least one of the beacon packets, theSTA acquires a vast amount of information. For example, the STA mayacquire knowledge of the following: a) the SAP's number of transmit andreceive directions during beaconing (i.e., M and N); b) the index ofSAP's preferred transmit direction (e.g., the beacon packet with thehighest LQI) from the SAP to the STA, referred to as the SAP's transmitdirection number m. Direction number m is acquired by the STA by sortingthe LQIs from the M beacon frames transmitted by the SAP in differentdirections and received by the STA using its Q receive directions. Thereare M×Q combinations in total, and one combination yields a best LQI.Alternatively, the STA may use the direction corresponding to the firstbeacon frame it successfully detects as its preferred direction; c) theindex of the STA's preferred receive direction when listening to theSAP. The STA's preferred receive direction is referred to as directionq; d) the list of devices that are members of the current BSS, and someor all of their capabilities in terms of PHY support (single carriersupport or OFDM support, data rates, number of transmit and receivedirections, etc.); e) the structure and duration of different fields ofthe superframe, such as the start time of CBP, duration of the CBP,superframe duration, etc.; and f) the time allocations of SPs in theCTAP.

Upon detection of the beacon, the STA goes through the associationprocess to become a member of the BSS. After association, the STA mayexchange data packets with another STA or with the SAP in accordancewith one of two procedures. In accordance with a first procedure, theSTA may access the contention-based period using a slotted ALOHAprotocol or a carrier sense multiple access with collision avoidanceCSMA/CA protocol in a manner similar to that specified in the IEEE802.11 protocol. In accordance with a second procedure, the STA requestsa service period (SP) from the SAP for the purpose of exchanging datapackets with another STA. If the request is accepted by the SAP, the SAPgrants access to the demanding STA and broadcasts a time allocation inthe beacon. The SAP may provide information about the source STA andaddress STA(s). The source and destination STAs may then exchange datapackets in the dedicated time allocated service period.

The association process involves transmitting an association requestfrom the STA to the SAP, and transmitting an association response fromthe SAP to the STA. This process may involve exchanges of many framesbefore the STA is considered to be associated. Furthermore, an STA mighthave to be authenticated prior to association. Authentication may bepart of the association process.

For networks such as IEEE 802.11, the SAP and STAs have a singletransmit antenna pattern, and the association process is relativelysimple and straightforward. IEEE 802.11 uses a CSMA/CA protocol. Toclarify the association process, a simple slotted ALOHA protocol isshown in FIG. 5B. The contention based period is divided into equal sizetime slots 560-1 to 560-T, where the slot duration encompasses theduration of an association request frame from the STA, plus the durationof a guard time, such as a first SIFS, plus the duration of anassociation response frame from the SAP, plus the duration of anotherguard time (such as a second SIFS). An STA may send a request only atthe beginning of a time slot. If the STA does not detect a responseafter a SIFS guard time, it regards the request as unsuccessful. Forexample, a collision may have occurred with a request sent by anotherSTA to the SAP during the same time slot. In this case, the STA mayretransmit the request in a future slot determined by some probabilitylaw calculation. Typically, a binary exponential back-off procedure isused. A STA draws a random number R₁ between 1 and W=2^(s) (where W isreferred to here as initial back-off window size), and transmits arequest frame in time-slot number R₁. If the request is unsuccessful,then the back-off window size is doubled and the STA draws anotherrandom number R₂ between 0 and 2^((q+1)) and retransmits the request intime-slot number R₁+R₂. Every time the request is unsuccessful, theback-off window size is doubled until it reaches a threshold 2^(S),after which the back-off window is constant. If the maximum back-offwindow is reached and the number of transmission trials exceeds apredetermined number, the station ceases its attempts to transmit theframe. On the other hand, if the transmission is successful, theback-off window size is reset back to 2^(s) for the next transmission.

For directional STAs and/or SAPs, the previously described slotted ALOHAprocedure may not perform well, especially when an STA does not knowwhich direction to use for transmissions to the SAP and the SAP does notknow which direction to use for reception. The same problem occurs withCSMA/CA, since good STA-to-SAP and SAP-to-STA directions are not knownat either side of the link. Furthermore, the hidden-node problem isworse, since stations cannot hear each other due to their directionalantenna patterns. In addition, the problem is more severe if thecontention based period is used for direction finding, authentication,association, service period (time allocation) request, data frameexchange between peer-to-peer STAs, and data frame exchanges between theSTA and SAP.

According to one aspect of the invention, the contention based period isdivided into two portions, a centralized contention based period (C-CBP)620 and a distributed contention based period (D-CBP) 630, such as shownin FIG. 6A. In the C-CBP 620, the SAP is a party to any communication.That is, communications occurs between the SAP and other STAs. In theD-CBP, the SAP is not necessarily part of every communication. That is,communications may occur between two STAs, and not the SAP.

In the following, a frame transmission from an STA to the SAP isreferred to as a request frame, and frame transmission from the SAP toan STA is referred to as a response frame.

In one aspect of the invention, the centralized contention based periodis divided into equal-size slots, such as shown in FIG. 6B. A time slotis further illustrated in FIG. 7A. At the beginning of a time slot, anSTA may transmit a request frame 710 to the SAP. After a guard interval(SIFS) 720, the SAP transmits a response frame 730 to the STA, and aguard interval SIFS follows before another request is allowed. The slotduration should be longer than the maximum duration of a request frame,plus a first SIFS, plus the maximum duration of a response frame, plus asecond SIFS. In the following, the combination of request frame andresponse frame is referred to as a transaction.

In FIG. 6B, the centralized based contention period (C-CBP) is dividedinto equal-size time slots 664-1-1 to 664-S-N. In the first time slot664-1-1, the SAP selects receive direction (pattern) number 1 andlistens for any request frame transmitted by an STA. If the SAP detectsa request frame, the SAP responds by transmitting a response frame aftera SIFS guard interval. In the second time slot 664-1-2, the SAP listensin receive direction number 2. This process is repeated for all N SAPreceive directions. In the N^(th) slot 664-N, the SAP employs receivedirection number N. In the (N+1)^(th) time slot 664-2-1, the SAP listensin receive direction number 1. In the (N+2)^(th) time slot 664-2-2, theSAP listens in direction number 2, and in the (2N)^(th) time slot664-2-N, the SAP listens while using receive direction number N. Insummary, the SAP uses its N receive directions in a cyclic manner.Therefore during the t^(th) time slot, the SAP uses its receivedirection number [(t−1) mod N]+1. A set of N consecutive time slotswherein the SAP cycles (i.e., sweeps) through its receive directions 1to N is referred to as a time cycle, such as illustrated by time cycles662-1 to 662-S.

According to one aspect of the invention, an SAP may specify fixedtime-cycle boundaries where the first time cycle boundary coincides withthe first time slot in the C-CBP. According to another aspect of theinvention, an SAP may leave the choice of time-cycle boundaries todifferent STAs. As an example, an STA might choose a time cycle as timeslots 664-1-2 to and including 664-2-1. That is, from an STA'sperspective, a time cycle comprises N consecutive time slots, such astime slots 1 to N, or time slots 2 to N+1, or time slots 3 to N+2, etc.

Before using the C-CBP, an STA acquires the beacon, such as describedpreviously. After beacon detection, an SAP acquires knowledge of itspreferred SAP transmit direction number m (i.e., the preferredSAP-to-STA transmit direction). The STA determines its preferred receivedirection number q by listening to the SAP. Therefore, before using theC-CBP, an STA is equipped with the SAP's preferred transmit direction mand its preferred receive direction n. The values m and q are thepreferred uplink pair of directions.

According to one aspect of the invention, an STA uses the same transmitdirection for each set of N consecutive time slots. This set ofN-consecutive time slots may (but not necessarily) be aligned with atime cycle, such as cycles 662-1 to 662-S. For example, if there is onlyone STA in the network, the STA may transmit a first request frame usingits transmit direction #1 in a time slot number 1 and then waits for aresponse. During this time slot, the SAP uses its receive directionnumber 1. If no response is detected by the STA, one of the reasons maybe that the combination of STA transmit direction number 1 and SAPreceive direction number 1 does not have enough LQI. The STA transmits asecond request frame in time-slot number 2 while still using itstransmit direction number 1, but the SAP employs its receive directionnumber 2. If the STA does not detect a response, it continuestransmitting request frames using the same transmit direction number 1for each of the N time slots or until a response is detected. If noresponse is received by the STA after transmitting in the N time slots,the STA uses transmit direction number 2 for the next N time slots oruntil a response is received. This process may be repeated for each ofthe STA's transmit directions or until a response is received. Such aprocess is referred to as a double sweep, wherein the SAP sweeps (i.e.,changes its direction) on a time-slot basis (i.e., the SAP's receivedirection changes every time slot). The STA sweeps on a cycle basis,that is, it changes its transmit direction every time cycle (i.e., Ntime slots). This double sweep is illustrated in FIG. 7B, which shows acycle of N time slots 760-1 to 760-N. During this cycle, the STA usesthe same transmit direction number PC during all time slots 760-1 to760-N, and the SAP changes its receive direction from one time slot toanother and cycles through all of its receive directions 1 to N.

According to another aspect of the invention, each request frame sent bythe STA comprises information regarding the SAP's preferred transmitdirection m. As described above, the STA determines the SAP's preferredtransmit direction from the beacon detection and monitoring stage. Oncethe SAP detects and decodes a request frame sent by an STA, itdetermines which transmit direction to use for transmitting the responseframe to the STA.

Once a response frame is detected by the STA, the STA and SAP have aworking pair of directions in both downlink and uplink. That is, the STAdetermines a working transmit direction toward the SAP and a preferredreceive direction when receiving from the SAP. The STA's workingtransmit direction is not necessarily the best direction. Rather, it maybe the first direction that results in a successful transaction(transmission/reception) with the SAP.

The previous aspect of the invention was explained in reference to asingle STA communicating with the SAP. When multiple stations contend toaccess the C-CBP, collisions occur. Therefore, there is a need for aback-off procedure that accounts for the directivity of the stations.

According to one aspect of the invention, an STA uses a set of N randomback-off numbers R(1), R(2), . . . , R(N) for determining the number ofback-off time slots (or equivalently, time cycles and time slots withinthe time cycles) before transmission corresponding to the N SAP receivedirections. The n^(th) random number R(n) indicates the number ofback-off time slots for a given target SAP receive direction number(i.e., when the SAP employs its receive direction number n). Therefore,the candidate time slots to be considered for back-off random numberR(n) are the time slots where the SAP's receive direction is denoted bydirection number n. For example, in reference to the numbering scheme inFIG. 6B, these candidate time slots include time-slots number n+k×N forany k (that is candidate time slots are time-slot numbers n+N, n+2×N,n+3×N, etc.). The random number R(n) determines the time-cycle number,and n determines the time-slot number within the selected time cycle.According to another aspect of the invention, a set of N binaryexponential back-off window sizes W(1), W(2), . . . , W(N) correspondingto the set of N SAP receive directions may be used. The back-off windowsizes are initially set to some initial values 2_(A(1)), 2_(A(2)), . . ., 2_(A(N)). That is, W(1)=2_(A(1)), W(2)=2_(A(2)), . . . ,W(N)=2_(A(N)).The initial values A(1), A(2), . . . , A(N) may be equal or they may bedifferent.

According to one aspect of the invention, an STA that needs to transmitin the C-CBP draws a set of N random numbers R(1), R(2), . . . , R(N),where R(n) is between 1 and W(n) for n=1, 2, . . . , N. As explainedabove, the random number R(n) determines the number of back-off timecycles, and the target time slot is the n^(th) time slot in cycle numberR(n). According to one aspect of the invention, the STA sorts the set ofrandom numbers in ascending order R[t(1)]≦R[t(2)]≦ . . . ≦R[t(N)], wheret(1) is the index of the smallest random number, t(2) is the index ofthe second smallest random number and so on. The first request frame istransmitted using transmit direction number t(1) in the t(1)^(th)time-slot number in cycle number R[t(1)]; that is, in time-slot number{R[t(1)]−1}×N+t(1) if the time slots are numbered 1, 2, 3, . . . fromthe boundary of the C-CBP, such as shown in FIG. 6B. If the transactionis not unsuccessful, the STA transmits a second request frame usingtransmit direction number t(2) in the t(2)^(th) time slot in cyclenumber R[t(2)]; that is, in time-slot number [t(2)−1]×N+1; and so on. IfR[t(n)] for some n is bigger than the number of slots available in theC-CBP, the transmission is delayed until the next C-CBP in the nextsuperframe. Each time a transaction is unsuccessful, the correspondingback-off window size is increased. According to one aspect of theinvention, the back-off window size is doubled every time thetransaction is unsuccessful in a given SAP receive direction until itreaches a maximum predetermined value, after which, it is kept unchangeduntil the transaction is successful or the STA ceases its attempts. Forexample, after the transmission of the first request frame in transmitdirection number t(1) in the t(1)^(th) time slot in cycle numberR[t(1)], if the transaction was unsuccessful, the STA doubles the valueof the back-off window size W[t(1)]. So if all of the first N trials areunsuccessful, the next set of N random numbers R(1), R(2), . . . , R(N)are drawn from 1 and 2*W(n) for n=1, 2, . . . N, and so on.

Aspects of the invention are further described with reference to FIG. 8.In this example, the SAP is assumed to have three receive directions(i.e., N=3), and the C-CBP 802 is divided into 24 time slots 806 to 860corresponding to 8 cycles 804-1 to 804-8, where each cycle 804-1 to804-8 contains three time slots corresponding to the three SAP receivedirections. As before, the SAP changes receive direction from one timeslot to another. For example, the SAP uses receive direction number 1 intime-slot number 1, receive direction number 2 in time-slot number 2,receive direction number 3 in time-slot number 3, receive directionnumber 1 in time-slot number 4, receive direction number 2 in time slotnumber 5, receive direction number 3 in time-slot number 6, and so on.The above aspect of the invention is explained with exemplary initialback-off window sizes W(1)=W(2)=W(3)=8. The STA draws three randomnumbers, R(1), R(2) and R(3). In the case depicted in FIG. 8, theserandom numbers are R(1)=5, R(2)=3, and R(3)=6. The STA sorts these threenumbers, as explained previously. In this example, t(1)=2, t(2)=1, andt(3)=3. The STA sends a first request frame using transmit directionnumber 2 (since t(1)=2) in the 2^(nd) time slot in cycle number 3 (sinceR[t(1)]=R(2)=3), that is, in time slot 820, which is time slot number 8(since {R[t(1)]-1}×N+t(1)={R(2)-1}×3+2=8). If the transaction isunsuccessful, the STA transmits a second request frame using transmitdirection number 1 (since t(2)=1) in the 1^(st) time slot in cyclenumber 5 (since R[t(2)]=R(1)=5), that is, in time slot 830, which istime-slot number 13 (since {R[t(2)]-1}×N+t(2)={R(1)-1}×3+1=8). If thesecond transaction is unsuccessful, the STA transmits a third requestframe using transmit direction number 3 (since t(3)=3) in the 3^(rd)time slot in cycle number 6 (since R[t(3)]=R(3)=6), that is, in timeslot 840, which is time-slot number 18 (since{R[t(3)]-1}×N+t(3)={R(3)−1}×3+3=18).

According to one aspect of the invention, upon a successful transaction,the STA uses the pair of working directions in which the successfultransaction occurred for future communication with the SAP. If, forexample, the third transaction was successful, then according to anotheraspect of the invention, the STA contends only in slots number 3*n forn=1, 2, 3, . . . . That is, if the STA needs to send a request frame inthe next superframe, the only candidate slots for possible transmissionsare time-slots number 3, 6, 9, 12, 15, 18, 21, and 24. The STA uses asingle back-off window size W=W(3) and a single random number R=R(3) toaccess the C-CBP. Furthermore, the STA uses the same transmit directionit used during the successful transaction. In summary, upon a successfultransaction, the STA has knowledge of the following: a) A workingtransmit direction toward the SAP, referred to as STA transmit directionnumber p; b) an SAP working receive direction, referred to as the SAPreceiver direction number n; c) the STA's preferred receive directionfrom the SAP, referred to as the STA receive direction number q; and d)the SAP's preferred transmit direction to the STA, referred to as theSAP transmit direction number m. The STA uses this pair of downlink anduplink directions for further communication with the SAP.

According to one aspect of the invention, after a successful transactionwith an STA, such as described above, the SAP stores a preferredtransmit direction to the STA and, optionally, a working receivedirection from the STA. Furthermore, the STA stores a preferred receivedirection from the SAP and a working transmit direction toward the SAP.

According to one aspect of the invention, the STA may use the C-CBP tofind a preferred downlink using the directional back-off proceduredescribed above. Upon a successful transaction with the SAP, the SAP hasa working downlink pair of directions (i.e., the STA working transmitdirection number p and the SAP working receive direction number n). Thispair of working downlink directions is not necessarily the best pair ofdirections. The SAP has N receive directions and the STA has P transmitdirections. In some aspects of the invention, the SAP surveys alldirection combinations (i.e., N×P directions) and the SAP measures theLQI for each combination (nc,pc), where nc=1 to N and pc=1 to P, to finda preferred pair of downlink directions. Upon a first successfultransaction, the STA will have tried the following combinations: a) Ntransactions in N times-slots in which the STA uses direction number 1and the SAP cycles through its N directions one at a time per time slot;b) N transactions in N times-slots in which the STA uses directionnumber 2 and the SAP cycles through its N directions one at a time pertime slot; c) p−1 transactions in N times-slots in which the STA usesdirection number p−1 and the SAP cycles through its N directions one ata time per time slot; and d) n time slots in which the STA usesdirection number p and the SAP cycles through directions 1 to n, wherein the last time slot, the working pair of directions (the STA transmitdirection number p and the SAP receive direction number m) were found.Therefore, the STA has gone through N×(p−1)+n transactions where onlythe last one was successful. The STA may choose to continue theprocedure, that is, the remaining N×P−[N×(p−1)×n] combinations ofdirections, using the above directional exponential back-off procedurein order to find a preferred downlink pair with a preferred LQI. If theSTA completes its trial of all N×P directions, this is a full doublesweep. Otherwise, if the STA stops at the first working downlink pair ofdirections, it is a partial successful double sweep.

According to one aspect of the invention, if an STA performs a fulldouble sweep, the SAP measures the LQI for each successful reception ofa request frame and sends the LQI as a feedback in one of the fields ofthe response frame. Furthermore, the STA may sort LQIs (either all LQIsor just those above a given threshold) and select at least one preferreddownlink pair for further transactions with the SAP. The preferreduplink pair is obtained from the beacon frames, as explained above, andmay not be part of the C-CBP direction search.

According to one aspect of the invention, if an STA performs a fulldouble sweep, the SAP measures the LQI for each successful reception ofa request frame and sends the LQI as a feedback in one of the fields ofthe response frame. Furthermore, the SAP may sort LQIs (either all LQIsor just those above a given threshold) and then provide feedback to theSTA. The STA may select the preferred downlink pair for furthertransactions with the SAP. The preferred uplink pair is obtained fromthe beacon frames, as explained above, and may not be part of the C-CBPdirection search.

According to one aspect of the invention, upon finding a working orpreferred pair of uplink and downlink directions (the SAP's transmitdirection number, m, the SAP's receive direction number, n, the STA'stransmit direction number, p, and the STA's receive direction number, q)an STA having a request frame to send may use a single back-off windowsize, W, and a single uniform number generator. According to one aspectof the invention, the STA generates a uniform random number R in therange 1 to W and uses the n^(th) time slot in time-cycle number Z totransmit the request frame using transmit direction number p. If this isthe first attempt by the STA to transmit the request frame, then Z=R. ifthis is not the first attempt by the STA to transmit the request frame,then Z=R+RACC, where RACC is the number of the time cycle used duringthe last unsuccessful transmission of the request frame. This procedureis part of the directional slotted ALOHA protocol and is used after theSTA has knowledge of at least the working or preferred transmitdirection to the SAP.

In the case in which an STA is moving, the preferred or working pair ofdownlink directions may change.

According to another aspect of the invention, after a full or partialsuccessful double sweep, an STA keeps a list of K downlink directionpairs (for example, a best pair and a second-best pair) and tracks andupdate the list by using the directional back-off algorithm in theappropriate time slots. In one aspect of the invention, 2 pairs aremaintained; a best pair (p₁,n₁) of downlink directions and a second-bestpair (p₂,n₂), where the first index (p₁ or p₂) refers to the STAtransmit direction number and the second index (n₁ or n₂) refers to theSAP receive direction number. The STA may determine the LQI of its besttransmit direction (measured by the SAP and sent back to the STA in theresponse frame) in every superframe and determine the LQI of the secondbest transmit direction (measured by the SAP and sent back to the STA inthe response frame) during every other superframe. So according toanother aspect of the invention, the tracking of the downlink directionpairs occurs at different update rates. When the STA updates the LQI ofthe best downlink direction pair (p₁,n₁), it may use the directionalexponential back-off algorithm. For example, the STA uses a back-offwindow size W1 and draws a random number R₁(1), where R₁(1) is between 1and W1. The STA sends a request frame in the n₁ ^(th) slot of cyclenumber R₁ (i.e., in slot number [R₁(1)−1]×N+n₁). The request framecontains information about the SAP's preferred transmit direction n1from the STA's perspective, information that is available to the STA asa result of decoding and tracking the beacon frames. The SAP receivesthe request frame using direction number n1, measures the LQI of therequest, and send the LQI back to the STA in the response frame. The SAPtransmits the response frame using transmit direction number n1, and theSTA receives the response using receive direction number q. Furthermore,the STA may update its list after each feedback or at the end of thedouble sweep. The request packet and response packet may be soundingpackets, which are specialized packets used for measuring and reportingchannel conditions and LQI. If the STA does not receive the responsepacket, or the response packet was not correctly decoded, the STAdoubles the back-off window size W1, draws a random number R₁(2), and asecond attempt is initiated by sending a request frame in the n₁ ^(th)time slot of cycle number [R₁(1)+R₁(2)], that is, in slot number[R₁(1)+R₁(2)−1]×N+n₁. The STA waits for a response, and if the responsepacket is decoded correctly, the STA receives the LQI (which was sent inthe response frame by the SAP) and updates its list of K downlinkdirection pairs. Each item of the list may simply contain the pair ofdirections (p,n) or the index p, and the corresponding LQI measured bythe SAP. In the event of a predetermined number of unsuccessfultransactions for the pair (p1,n1), the STA may remove the pair (p1,n1)from the list and select (p2,n2) as an alternative or temporarypreferred pair until a better pair is found.

According to one aspect of the invention, if during or after updatingthe STA's list of K downlink direction pairs, a better downlinkdirection pair is discovered, the STA may select the better downlinkdirection pair for further transactions with the SAP.

According to one aspect of the invention, the C-CBP is used for at leastone of STA authentication, association, transmit and/or receivedirection finding, direction tracking, control frames, service periodreservation, command frames, management frames, and data frames, wherein all cases, the communication is between the STA and the SAP. An STAmay use the directional slotted-ALOHA protocol in the C-CBP to exchangedata frames with the SAP. However, the length of the data frames shouldbe selected such that a transaction does not exceed the slot boundary.If the frame is too long, it should be adapted to fit within a time slotalong with the response and two SIFS. As another example of a datatransaction, the request frame may be a data frame and the responseframe may be an immediate acknowledgment. For association, the requestframe may be an association request and the response frame may be anassociation response. Peer-to-peer communications in which neither peeris an SAP is preferably performed in the D-CBP. As another example, therequest frame can be a service period reservation request by an STA, andthe response frame sent by the SAP may be the SAP denial or acceptanceof the service period reservation. Each task (such as association,authentication, service period reservation, etc.) may require more thana simple exchange of two frames (i.e., the request frame and responseframe). Rather, a task may require multiple request-response frames.Direction finding may be performed using a full double sweep or apartial successful double sweep. The direction acquisition (finding) canbe performed as part of authentication and/or association, or it may beperformed independently. When performed independently, it is preferablydone before any other task in the C-CBP, such as before authentication,association, and data exchange. If the direction acquisition isaccomplished as an independent STA task, the request and response framesused during the sweep may be specialized sounding packets.Alternatively, if the direction acquisition is part of authentication,then the request and response frames are authentication request andresponse frames.

According to another aspect of the invention, a directional cycle-basedALOHA method is employed wherein the directional exponential back-off iscycle-based rather than slot-based. Specifically, an STA may use asingle back-off window size W and a single random number R. The STAgenerates a random number R₁ (1≦R₁≦W) that is used for the first Ncandidate transmissions in N time slots in cycle number R. The STAtransmits a first request frame using transmit direction number 1 in thefirst time slot of time-cycle number R. During the first time slot, theSAP employs its first receive direction. If the STA does not receive aresponse, the transaction is unsuccessful, so the STA transmits a secondrequest frame in transmit direction number 1 in the second time slot oftime-cycle number R. The SAP employs its second receive direction in thesecond time slot, and the STA listens for a response. This process maybe repeated for all N time slots within the time-cycle number R. Ifthere are no successful transactions, the STA doubles its back-offwindow and generates a second random number R₂ (1≦R₂≦2W). The randomnumber R₂ is used for the second set of N scheduled transmissions in Ntime slots in cycle number R₁+R₂, and the STA repeats its transmissionprocess using transmit direction number 2. This process may be repeatedfor all the STA transmit directions.

An STA may use both directional cycle-based ALOHA and directionalslot-based ALOHA. As an example, an STA may use cycle-based ALOHA forinitial direction acquisition (e.g., direction-finding) during a partialor full double sweep. In this case, an STA may finish the double sweepin P (non-consecutive) cycles such that within each time cycle, the STAemploys a fixed transmit direction and the SAP sweeps over all of itsreceive directions, one receive direction per time slot. When apreferred downlink pair of directions is found, the pair may be used forauthentication, association, and further data transfer with adirectional slotted ALOHA protocol, as previously described.

FIG. 9A illustrates an exemplary method 900 for encoding the centralizedcontention based period (C-CBP) in a beacon frame. A beacon is generatedby an SAP 902, and the C-CBP information and the superframe timing andstructure are encoded in the beacon 904 before transmission 906.

FIG. 9C illustrates an exemplary method 940 that may be performed by anSTA to process a received beacon frame. An STA receives a beacon frame942, demodulates the beacon frame 944, and extracts C-CBP and superframeinformation 946.

FIG. 10A illustrates an exemplary method 1000 for processing a frame fortransmission by an STA using a directional slotted ALOHA protocol. AnSTA prepares a data frame for transmission 1002. The STA generates a setof N uniform random variables R(1) to R(N) 1004, where N is the numberof SAP receive directions and R(n) is in the range of 1 to W(n), whereW(n) is the n^(th) back-off window size and n=1 to N. The STA sorts thelist of random numbers in increasing order 1006. For example, the sortedlist may be expressed by R[T(1)]≦R[T(2)≦ . . . ≦R[T(N)]. The STAtransmits the frame 1008 in the T(NC)^(th) time slot of cycle numberR[T(NC)] for at least some NC=1 to N. FIG. 10B illustrates an apparatusconfigured for performing the steps shown in FIG. 10A.

FIG. 10C illustrates an exemplary method 1040 for processing a requestframe by an SAP using a directional slotted ALOHA protocol. An SAPreceives a request frame 1042. The SAP detects and decodes the requestframe 1044 to extract information encoded in the frame regarding thetransmit direction (e.g., a transmit direction index) it should use forsending response frames to the STA. The SAP stores the STA ID and thetransmit direction index 1046. FIG. 10D illustrates an apparatusconfigured for performing the steps shown in FIG. 10C.

FIG. 11A illustrates an exemplary method 1100 for frame transmission byan STA using a directional slotted ALOHA protocol wherein the STA hasknowledge of the working direction toward the SAP. The STA prepares aframe for retransmission 1102. The STA obtains information 1104 (such asfrom a previous partial or full double sweep) regarding its transmitdirection number (PC), SAP receive direction number (NC), SAP transmitdirection number (MC), the back-off window size W, and cycle number(RAcc) where the last transmission from the STA toward the SAP with theSAP in receive direction number (NC) has occurred. The STA encodes theSAP's transmit direction (MC) in the request frame 1106 in order toinform the SAP that this is the direction to use to transmit theresponse frame. The STA generates a uniform random number 1108 in therange 1 to W. The STA transmits the request frame 1110 using transmitdirection number (PC) in the NC^(th) time slot of time-cycle numberR+RACC. FIG. 11B illustrates an apparatus configured for performing thesteps shown in FIG. 11A.

FIG. 12A illustrates an exemplary method 1200 according to one aspect ofthe invention. An SAP receives a request frame from an STA 1202 whereinthe STA has encoded the transmit direction NC that the SAP should usefor transmitting frames to the STA. The SAP decodes the request frame1204 to obtain the transmit direction NC it should use for respondingback to the STA. The SAP generates a response frame 1206 and transmitsthe response frame 1208 using transmit direction number NC. FIG. 12Billustrates an apparatus configured for performing the steps shown inFIG. 12A.

FIG. 13A illustrates an exemplary method 1300 performed by an STA tofind at least one working or preferred downlink direction. The STAperforms a full or partial double sweep using the directional slottedALOHA protocol 1302. The STA determines preferred or working downlinkdirections from response frames received from the SAP 1304. The STAtransmits the preferable or working downlink directions 1306 to the SAP.FIG. 13B illustrates an apparatus configured for performing the stepsshown in FIG. 13A.

The various operations of methods described above may be performed byany suitable means capable of performing the corresponding functions.The means may include various hardware and/or software component(s)and/or module(s), including, but not limited to a circuit, anapplication specific integrated circuit (ASIC), or processor. Generally,where there are operations illustrated in Figures, those operations mayhave corresponding counterpart means-plus-function components withsimilar numbering. For example, blocks 902-906, 942-946, 1002-1008,1042-1046, 1102-1110 1202-1208, and 1302-1306, illustrated in FIGS. 9A,9C, 10A, 10C, 11A, 12A and 13A correspond to circuit blocks 922-926,962-966, 1022-1028, 1062-1066, 1122-1130, 1222-1228 and 1322-1326illustrated in FIGS. 9B, 9D, 10B, 10D, 11B, 12B and 13B.

As used herein, the term “determining” encompasses a wide variety ofactions. For example, “determining” may include calculating, computing,processing, deriving, investigating, looking up (e.g., looking up in atable, a database or another data structure), ascertaining and the like.Also, “determining” may include receiving (e.g., receiving information),accessing (e.g., accessing data in a memory) and the like. Also,“determining” may include resolving, selecting, choosing, establishingand the like.

The various operations of methods described above may be performed byany suitable means capable of performing the operations, such as varioushardware and/or software component(s), circuits, and/or module(s).Generally, any operations illustrated in the Figures may be performed bycorresponding functional means capable of performing the operations.

The various illustrative logical blocks, modules and circuits describedin connection with the present disclosure may be implemented orperformed with a general purpose processor, a digital signal processor(DSP), an application specific integrated circuit (ASIC), a fieldprogrammable gate array signal (FPGA) or other programmable logic device(PLD), discrete gate or transistor logic, discrete hardware componentsor any combination thereof designed to perform the functions describedherein. A general purpose processor may be a microprocessor, but in thealternative, the processor may be any commercially available processor,controller, microcontroller or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thepresent disclosure may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in any form of storage medium that is knownin the art. Some examples of storage media that may be used includerandom access memory (RAM), read only memory (ROM), flash memory, EPROMmemory, EEPROM memory, registers, a hard disk, a removable disk, a CDROMand so forth. A software module may comprise a single instruction, ormany instructions, and may be distributed over several different codesegments, among different programs, and across multiple storage media. Astorage medium may be coupled to a processor such that the processor canread information from, and write information to, the storage medium. Inthe alternative, the storage medium may be integral to the processor.

The methods disclosed herein comprise one or more steps or actions forachieving the described method. The method steps and/or actions may beinterchanged with one another without departing from the scope of theclaims. In other words, unless a specific order of steps or actions isspecified, the order and/or use of specific steps and/or actions may bemodified without departing from the scope of the claims.

The functions described may be implemented in hardware, software,firmware or any combination thereof. If implemented in software, thefunctions may be stored as one or more instructions on acomputer-readable medium. A storage media may be any available mediathat can be accessed by a computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code in the form of instructions or datastructures and that can be accessed by a computer. Disk and disc, asused herein, include compact disc (CD), laser disc, optical disc,digital versatile disc (DVD), floppy disk, and Blu-ray® disc where disksusually reproduce data magnetically, while discs reproduce dataoptically with lasers.

Thus, certain aspects may comprise a computer program product forperforming the operations presented herein. For example, such a computerprogram product may comprise a computer readable medium havinginstructions stored (and/or encoded) thereon, the instructions beingexecutable by one or more processors to perform the operations describedherein. For certain aspects, the computer program product may includepackaging material.

Software or instructions may also be transmitted over a transmissionmedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition oftransmission medium.

Further, it should be appreciated that modules and/or other appropriatemeans for performing the methods and techniques described herein can bedownloaded and/or otherwise obtained by a user terminal and/or basestation as applicable. For example, such a device can be coupled to aserver to facilitate the transfer of means for performing the methodsdescribed herein. Alternatively, various methods described herein can beprovided via storage means (e.g., RAM, ROM, a physical storage mediumsuch as a compact disc (CD) or floppy disk, etc.), such that a userterminal and/or base station can obtain the various methods uponcoupling or providing the storage means to the device. Moreover, anyother suitable technique for providing the methods and techniquesdescribed herein to a device can be utilized.

It is to be understood that the claims are not limited to the preciseconfiguration and components illustrated above. Various modifications,changes and variations may be made in the arrangement, operation anddetails of the methods and apparatus described above without departingfrom the scope of the claims.

The techniques provided herein may be utilized in a variety ofapplications. For certain aspects, the techniques presented herein maybe incorporated in a base station, a mobile handset, a personal digitalassistant (PDA) or other type of wireless device that operate in UWBpart of spectrum with processing logic and elements to perform thetechniques provided herein.

1. A digital computer network comprising: a first transmitter componentresiding on a first wireless device employing a sequence of time slotspaired with a plurality of transmit directions for transmitting at leastone request frame to a second wireless device; a first receivercomponent residing on the first wireless device configured for receivingat least one response frame from the second wireless device; a secondreceiver component residing on the second wireless device employing adifferent one of a plurality of receive directions for each of thesequence of time slots to listen for the at least one request frametransmitted by the first transmitter; a second transmitter componentresiding on the second wireless device configured for transmitting atleast one response frame to the first wireless device; and a digitalcomputer system comprising a memory for storing instructions and aprocessor for executing the instructions for selecting a preferred setof uplink and downlink directions for further communication between thefirst wireless device and the second wireless device.
 2. The digitalcomputer network of claim 1, wherein a time cycle comprises a pluralityN of consecutive time slots, where N equals the plurality of receivedirections; and wherein the second receiver component is configured forcycling through the plurality of receive directions in each time cycle.3. The digital computer network of claim 2, wherein each time cycle hasa time-cycle boundary selected by at least one of the first wirelessdevice and the second wireless device.
 4. The digital computer networkof claim 2, wherein each time cycle has a time-cycle boundary selectedby at least one of the first wireless device and the second wirelessdevice.
 5. The digital computer network of claim 1, wherein thepreferred set comprises a preferred transmit direction for transmittingfrom the second wireless device to the first wireless device, apreferred receive direction for receiving transmissions from the secondwireless device to the first wireless device, and a preferred transmitdirection for transmitting from the first wireless device to the secondwireless device.
 6. The digital computer network of claim 1, wherein thedigital computer system selects the preferred set based on measurementsof link quality.
 7. The digital computer network of claim 1, wherein thedigital computer system performs at least one of a full double sweep anda partial double sweep of all possible uplink and downlink directionpairs when selecting the preferred set.
 8. The digital computer networkof claim 1, wherein the preferred set comprises at least one of a set ofdirections having an optimal link quality and a set of directions havinga link quality above a predetermined threshold.
 9. The digital computernetwork of claim 1, wherein selecting the preferred set comprisesperforming at least one of a full double sweep and a partial doublesweep of all possible uplink and downlink direction pairs.
 10. Thedigital computer network of claim 1, wherein selecting the preferred setcomprises selecting an uplink pair from beacon frames and selecting adownlink pair during a C-CBP direction search.
 11. The digital computernetwork of claim 1, wherein the digital computer system is configuredfor maintaining a plurality of downlink direction pairs, updating a linkquality indicator measurement for each of the plurality of downlinkdirection pairs, and updating at least one of the set of uplink anddownlink directions.
 12. The digital computer network of claim 1,configured for performing at least one of directional slotted ALOHA anddirectional cycle-based ALOHA.
 13. The digital computer network of claim1, configured for performing at least one of authentication,association, direction finding, direction tracking, communicatingcontrol frames, service period reservation, communicating commandframes, communicating management frames, and communicating data frames.14. A digital computer system, comprising: a transmitter configured forselecting a sequence of time slots paired with a plurality of transmitdirections for transmitting at least one request frame to at least onewireless device, the at least one wireless device employing a differentone of a plurality of receive directions for each of the time slots; areceiver configured for listening for at least one response frametransmitted by the at least one wireless device; and a memory forstoring instructions and a processor for executing the instructions forselecting a preferred set of uplink and downlink directions for furthercommunication with the at least one wireless device.
 15. The digitalcomputer system of claim 14, wherein selecting the sequence furthercomprises employing a first transmit direction for a first plurality Nof the time slots, and a second transmit direction for a secondplurality N of the time slots, where N equals the plurality of receivedirections.
 16. The digital computer system of claim 14, wherein thetransmitter is configured for employing a different transmit directionfor each of a plurality of time cycles when transmitting the at leastone request, wherein each of a plurality of time cycles comprises aplurality N of consecutive time slots.
 17. The digital computer systemof claim 14, wherein each of the time slots comprises a request frameslot, a first guard interval, a response frame slot, and a second guardinterval.
 18. The digital computer system of claim 14, wherein thetransmitter employs a set of back-off numbers corresponding to receivedirections for determining back-off times for transmitting requestframes.
 19. The digital computer system of claim 14, wherein thetransmitter employs an algorithm for selecting transmit directions andtime cycles for transmitting request frames.
 20. The digital computersystem of claim 14, configured to perform at least one of directionalslotted ALOHA and directional cycle-based ALOHA.
 21. A digital computersystem, comprising: a receiver configured for employing a different oneof a plurality of receive directions for each of a sequence of timeslots to listen for at least one transmitted request frame from awireless device; a transmitter configured for transmitting at least oneresponse frame in response to a received request frame; and a memory forstoring instructions and a processor for executing the instructions forselecting a preferred set of uplink and downlink directions for furthercommunication with the wireless device.
 22. The digital computer systemof claim 21, wherein a time cycle comprises a plurality N of consecutivetime slots, where N equals the plurality of receive directions; andwherein listening for the at least one transmitted request framecomprises cycling through the plurality of receive directions in eachtime cycle.
 23. The digital computer system of claim 21, wherein therequest frame comprises a preferred transmit direction.
 24. The digitalcomputer system of claim 21, wherein selecting the preferred set furthercomprises measuring an uplink link quality indicator and transmittingthe uplink link quality indicator to the wireless device.
 25. A digitalcomputer system, comprising: a transmitter configured for transmitting arequest frame to a receiver employing a plurality of receive directionsto listen for the request frame; and a memory for storing instructionsand a processor for executing the instructions for: generating aplurality of time cycle numbers, each of the time cycle numbers beingassociated with one of the receive directions and having a value withina predetermined back-off window size; sequentially organizing theplurality of time cycle numbers with respect to their values forproducing a sequence of time cycle numbers; and generating a sequence oftime slot numbers from the sequence of time cycle numbers and theplurality of receive directions, the sequence of time slot numbers beingused by the transmitter to select time slots for transmitting therequest frame.